The present invention relates to a raw powder for sintering, and also relates to a microparticulate low-alloyed steel for sintering or other such raw powder for sintering that has a high sintered density and has been modified to obtain uniform characteristics, or a granulated powder for sintering obtained by granulating the raw powder for sintering, and to a sintered compact obtained using the raw powder for sintering or the granulated powder for sintering.
When sintered articles are manufactured, the properties required of the sintered products include high density, dimensional precision, and characteristics such as tensile strength, hardness, fatigue strength, impact strength, abrasion resistance, and other such characteristics, as well as uniformity, but it is not easy to simultaneously achieve the desired density, dimensional precision, and other characteristics.
Typical known methods for obtaining a conventional sintered compact of low-alloyed steel include premix methods and prealloy methods.
A premix method is a method of uniformly mixing iron powder and metal powder or alloy powder, molding the resulting mixture, and then heating and sintering the mixture to obtain a solid solution from the added elements. This method has difficulties in that the added metal powder or alloy powder mixed into the iron powder separates or segregates due to differences in relative density in the steps up to molding, or that the diffusion of added elements during sintering is inadequate and does not progress uniformly, and problems with product quality occur in that there are fluctuations in the strength or dimensions of the sintered compact due to the nonuniformity of the sintered compact.
Metal injection molding is known as a method of obtaining a sintered compact with a high sintered density and uniform properties in the molding step. In this method, a mixture of carbonyl iron powder, carbonyl nickel powder, or another such carbonyl metal powder with a small powder grain size is commonly used. However, these carbonyl metal powders are manufactured by thermal decomposition of a metal carbonyl, and such manufacture requires high energy and is expensive, and also the metal carbonyl itself is poisonous and has therefore been the cause of environmental problems. Also, carbonyl nickel powder has poses health problems when absorbed into the body. Furthermore, because of the aspects of carbonyl powder, there have also been problems of low molding density and poor dimensional precision compared with atomized powder.
A prealloy method is a method of using a powder manufactured by atomization or the like from alloyed steel wherein Ni, Cu, Mo, or other such necessary alloying elements have been dissolved in iron in advance. Since the necessary alloying elements are dissolved in iron in advance, this method is free of problems such as those encountered in the premix method and metal injection molding method.
As an example of obtaining raw powder for sintering or a sintered compact by prealloy methods, Japanese Patent Application Laid-open No. H6-57301 discloses the concept of prealloying Cr and Mo and alloying Ni or Cu by composite alloying to obtain a raw powder for sintering that combines high dimensional precision, high strength, and high fatigue properties, that has little dimensional variation and strength fluctuation, and that is suitable for manufacturing sintered articles.
Also, Japanese Patent Application Laid-open No. H11-302787 discloses that high-strength sintered articles can be manufactured from water-atomized iron powder comprising 1.0 to 10.0 wt % of Cu as a prealloying element, with Fe and unavoidable impurities constituting the balance.
However, with these prealloyed conventional atomized powders, the primary phase of the metal structure is the austenite phase at a temperature of 1200° C. or greater, which is the sintering temperature. Therefore the diffusion rate is low, which causes problems in that sintering is reduced and the sintered density cannot be increased, as well as problems in that the crystal grains tend to become coarser, making it difficult to obtain a sintered compact with stable characteristics.
Furthermore, regarding the effect of the grain size of the powder, there have also been problems with the post-sintering depth of carburized hardening becoming unstable because of low sintered density and a large number of closed pores when the powder has a large mean grain size.
Yet another problem is that although the sintering may be satisfactory and the sintered density may be improved, the fluidity is still inferior. Therefore, problems have been encountered in the sense that poor moldability is achieved with compression molding methods, and, in particular, that uniform filling is difficult to achieve in the case of products with complicated shapes.
Also, since the molding density is low, dimensional precision of the sintered compact has been poor and there have been limits on the grain size that can be used. Therefore, there have also been limits on the sintered density of the resulting sintered compact.
Furthermore, sintered compacts with low sintered densities have had problems in that strength, toughness, and other such characteristics are poor and vary widely, the product surface has low glossiness and a poor appearance, and product quality is compromised, so there have been restrictions on the applicable range of grain sizes for the raw powder for sintering.
There are also examples of increasing the strength of a sintered compact without increasing its density (for example, Japanese Patent Application Laid-open No. H6-65693), but even in such cases it is vital to increase the density of the sintered compact in order to obtain strength equal to that of products obtained by melting.
The present invention was achieved in view of circumstances such as those described above, and an object thereof is to provide raw powder for sintering or granulated powder for sintering whereby a sintered compact with a high sintered density can be obtained at a low manufacturing cost.
Another object is to provide raw powder for sintering or granulated powder for sintering whereby a sintered compact having stable characteristics and a satisfactory post-sintering depth of carburized hardening can be obtained.